1. Field of the Invention
The instant invention relates to a full mold casting process for a differential case member in which a complete positive model, made of a thermally decomposable material, is placed into a molding box filled with sand. The sand may be compacted by any suitable method such as shaking, and the casting metal is poured on the thermally decomposable model. The model is decomposed by the casting heat.
2. Description of Related Art
To produce castings, molds consisting of two halves are generally used. However, this division of the molds is only possible within certain dimensional tolerances, and as a consequence, defects may occur within the mold joint due to offset burrs, sand washouts, metal penetrating into the sand mold, charred portions of sand, and the like. All these defects appear on the cast and must be eliminated by grinding, stripping, knocking, sawing, or the like.
To avoid these extensive manual tasks, a so-called evaporative or full mold casting process has been proposed whereby a pattern is formed of an evaporable foam material, such as polystyrene, and is identical to the configuration of the metal part to be cast. The pattern is placed in a mold and a flowable material such as sand is introduced into the mold and surrounds the pattern as well as filling the cavities in the pattern. In the casting process, molten metal is introduced into the mold and the heat of the molten metal will vaporize the foam material with the vapor being trapped within the interstices of the sand, while the molten metal will fill the voids created by vaporization of the pattern to provide a cast metal part which is identical in configuration to the evaporable foam pattern. When such an undivided model is used, a casting without burrs is produced.
It has been found, however, that not all types of models can be molded in this manner and it is especially difficult to satisfactorily embed moldings with dome-shaped cavities, such as one-piece differential case members and similar shapes, in the sand because the sand does not rise to fill these cavities. It is, therefore, a disadvantage of this known full mold casting process that such moldings cannot be molded and cast in this process in an efficient and effective manner but must be produced with batch cores or in the conventional molding process, in several parts. As an alternative, attempts have been made to distribute sand by an extreme amount of shaking of the molding flask. This shaking can cause damage to the foam pattern causing dimensional inaccuracies. In addition, the excess shaking will also increase the cost of the part due to the longer cycle times for filling the flask and excessive wear on the molding machine. All molds in which the sand would have to rise into such cavities as well as into communicating pipes are, therefore, inefficient and ineffective with the known full mold casting process for one-piece differential case member and the like.
The need therefore exists for a method of making a differential case or other similar article using lost foam casting whereby sand is allowed to flow into the interior of the part where the sand would otherwise not have access.
It is the object of the instant invention to improve the full mold casting process so that moldings with crucial cavities can also be molded and filled safely by the molding sand.
A further object consists in distributing the molding sand in a controlled manner without excessive shaking.
The objects are attained through the invention in that the part is formed with holes on the dome-shaped portion, i.e., the flange and button sides of the differential case, that allow sand to freely flow into the interior of the casting.
In an advantageous further object of the process the weight of the part is reduced and damage to the foam pattern is reduced or eliminated. As a result, dimensional inaccuracies of the part are reduced or eliminated. The invention makes it possible to achieve reliable density of the sand on all sides around the molding pattern without danger of the sand being loosened.
By introducing the flow-through holes into the casting model, preferably through the flange and button sides, all cavities of the casting model are completely filled with sand and the latter is compressed against the model. The rising of the sand in the model cavity is facilitated by the fact that the filling level of the molding sand is kept nearly constant near the model cavity to be filled until the cavity is completely filled.
The model is preferably enclosed in sand, which does not include a binding agent in the sand. The number, shape, orientation and size of pass-through holes can vary so long as they allow access to the interior of the casing or dome-shaped portions.
In accordance with the objects described above, the invention is a method for making a differential case metal casting or similar article and the resulting product. The method comprises the following steps: (a) providing an in situ destroyable foam pattern to form an integral one-piece differential case casting of a desired shape. The pattern has a hollow body including a substantially annular central wall section and axially spaced side wall sections adjacent to opposite ends of the central wall section forming an internal cavity within the hollow body of the foam pattern. The pattern (and, thus, the differential case) includes two sets of openings on the side wall sections of the hollow body of the pattern adjacent to the respective opposite end of the central wall section. In the subsequent step (b), the mold pattern is embedded in unbonded sand so that the unbonded sand is at least partially introduced into the cavity in the mold pattern through the plurality of openings in said hollow body. The new method of the present invention allows sand to flow freely into the cavity of the mold pattern, without the need for substantial shacking of the molding flask.